Urea distribution systems are often used in selective catalytic reduction (SCR) processes. The urea, in conjunction with an SCR catalyst, reduces NOx, for example in an exhaust stream from an internal combustion engine. An example SCR catalyst may treat the exhaust of a diesel engine on a motor vehicle, which produces a variable amount of NOx in response to the duty cycle variations of the vehicle. In such processes, the amount of urea required for effective NOx reduction depends on the exhaust flow rate and the concentration of NOx in the exhaust stream. The amount of urea injected may be highly variable or transient during certain operating periods, while during other periods the urea injection may be discontinued. The loss of urea injection capability during operations experiencing highly variable NOx output may be especially detrimental to meeting emissions requirements or other operational requirements.
A urea distribution system may utilize a pump to provide pressurized urea to an injector for injecting into the exhaust of an engine. A diaphragm pump is a desirable alternative for such a pump. However, diaphragm pumps suffer from several drawbacks, including a limited ability to self-prime. When a diaphragm pump loses prime, the limited compression ratio of the pump prevents the exit valve from opening into a pressurized downstream line, causing the pump to experience “vapor lock.”
Presently known urea distribution systems manage potential vapor locking with a return line recirculating from the pump inlet to the urea tank. With a return line installed, a pump that has lost prime can recirculate air into the urea tank and receive liquid at the inlet, allowing the pump to re-prime. The presence of a return line requires additional plumbing of lines, including additional installation space, additional potential leakage sites, and a more complex response of the system to the vibration environment. Further improvements in this area are desirable.